The invention relates to a process for the preparation of isocyanates by phosgenation of primary amines and to a mixing unit of the rotor-stator type which is suitable for mixing two or more different substances, including the educts phosgene and primary amine. The mixing unit of the rotor-stator type is a mixer reactor and is suitable for mixing, initiating and carrying out a reaction of at least two flowable substances which can have considerable differences in viscosity. This mixer reactor is particularly suitable for the preparation of mono- or polyisocyanates by reacting mono- or polyamines with phosgene dissolved in an organic solvent.
Rotor-stator mixers in general comprise at least one rotor provided with mixing elements and a stator surrounding the rotor which is equipped with elements which break the flow. Such rotor-stator mixers are generally known. In this context, the rotor rotates at a high speed of rotation, while the stator remains in a fixed position. By the movement of the rotor, the liquid in the annular gap between the rotor and the stator is mixed with high turbulence. In the case of non-miscible liquids, one of the two liquid is finely dispersed in the other by the high energy input. In this context, the dispersion formed is more finely dispersed as the speed of rotation increases (i.e. is higher), and therefore, the energy input is higher. Due to the high speed of rotation of the rotor, a large amount of energy is introduced into the liquid and converted into heat during the mixing. As a result, an increase in the temperature of the liquid mixture occurs in the course of the mixing operation.
It is known to carry out reactions which start rapidly, such as the preparation of mono- or polyisocyanates by reaction of mono- or polyamines with phosgene, in a mixer reactor of the rotor-stator type (and optionally additional subsequent reaction apparatuses) which comprises a substantially rotationally symmetric housing, wherein the housing has a substantially rotationally symmetric mixing chamber with separate inlets for at least two substances and an outlet, wherein the inlet for the first substance is provided in the axis of the mixing chamber and the inlet for the at least second substance is constructed in the form of a plurality of openings arranged rotationally symmetrically to the mixing chamber axis. See, for example, EP 291 819 B1 (believed to correspond to U.S. Pat. No. 4,851,571) and EP 291 820 B1 (corresponds to U.S. Pat. No. 4,915,509).
According to the prior art, mixing units in which each inlet which is constructed in the form of openings arranged rotationally symmetrically to the mixing chamber axis is assigned a pin which can be displaced in the axial direction. By displacing the pin axially, the opening can be penetrated by the pin, and thus freed from any deposits present. It is preferably to displace each pin into the inlet opening, either in the event of an increase in pressure in the feed line or periodically. See, for example, EP 830 894 B1 (corresponds to U.S. Pat. No. 5,931,379).
It is likewise known to carry out reactions which start rapidly, such as the reaction of mono- or polyamines with phosgene, in mixing units which comprise a substantially rotationally symmetric housing, wherein the housing has a substantially rotationally symmetric mixing chamber with separate inlets for at least two substances and an outlet, wherein the inlet for the first substance is provided in the axis of the mixing chamber and the inlet for the at least second substance is radial or lateral with respect to the axis of the mixing chamber and the mixing chamber has no moving parts. See, for example, EP 322 647 B1 (corresponds to U.S. Pat. No. 5,117,048) and WO 2002/002217 A1.
It is moreover known to carry out reactions which start rapidly, such as the reaction of mono- or polyamines with phosgene, in mixing units which comprise a substantially rotationally symmetric housing, wherein the housing has a substantially rotationally symmetric mixing chamber with separate inlets for at least two substances and an outlet, wherein at least both inlets are arranged radially to the axis of the mixing chamber. See, for example, DE 10 034 621 A1, U.S. Pat. No. 4,886,368, and DE 42 20 239 C2.
The quality of the mono- or polyisocyanates prepared in such apparatuses depends decisively on the quality and speed of mixing of the at least two flowable substances. In this context, maintaining a uniform mass flow through the mixer reactor plays a decisive role in particular, since backmixing of substances which have already reacted with one another into the substance streams of the unreacted starting substances can thereby be prevented.
A general criterion of the quality of a mixing apparatus is the mixing time which can be achieved with the particular apparatus. The mixing time of a mixing device which is employed for initiating a rapid reaction, such as the preparation of mono- or polyisocyanates by reaction of mono- or polyamines with phosgene dissolved in an organic solvent; is conventionally 0.0001 s to 5 s, preferably 0.0005 s to 4 s, particularly preferably 0.001 s to 3 s (see, for example, DE 10 2005 014846 A1). Mixing time as used herein is to be understood as meaning the time which passes from the start of the mixing operation until 97.5% of the fluid elements of the mixture obtained have reached a specific mixture fraction. This mixture fraction shall not deviate more than 2.5% from the theoretical final value of the mixture obtained when the state of perfect mixture is assumed. The concept of the mixture break is explained e.g. in J. Warnatz, U. Maas, R. W. Dibble: Combustion, Springer Verlag, Berlin Heidelberg N.Y., 2006, 4th edition, p. 136-137.
The quality of the thorough mixing and the completeness of the prevention of backmixing can be seen concretely from several criteria.
By inadequate mixing, caking up to blockages occurs over the course of time within the inlet openings for the at least second substance, so that the introduction of equal material flowing through all openings is disturbed. This impairs the flow properties through the mixer reactor, such that backmixing increasingly occurs.
The size and size distribution of the amine hydrochloride and carbamoyl chloride particles which form during the reaction, the size of which should be in the nanometer to micrometer range, are a further criterion for the quality of the thoroughness of the mixing. The formation of relatively large amounts of these solids is to be prevented, since formation of large and agglomerated amine hydrochloride particles may occur as a result, the phosgenation of which, as described in the literature, is very slow. (see, for example, WO 2004/056756 A1).
The color or the viscosity of the mono- or polyisocyanates obtained is also a further criterion of the quality of the thoroughness of mixing, since if all side reactions are suppressed completely, a colorless and low-viscosity product may be obtained.
The content of free isocyanate groups (NCO value) in the product obtained is a further criterion of the quality of the thoroughness of mixing, since the content remains low if thorough mixing is inadequate, and drops further if backmixing exists. The content of free isocyanate groups can be determined in a simple manner as the so-called NCO value. The NCO value is determined by reaction of the isocyanate with excess dibutylamine to give the corresponding urea and back-titration of the non-consumed amine with hydrochloric acid standard solution. A high NCO value is preferred for industrially suitable mono- and polyisocyanates.
In the known mixing units of the rotor-stator type, mixing is carried out by a procedure in which the first substance metered axially flows outwards due to the centrifugal form of the first rotor disc and is thereby charged with the second substance introduced, and the two substance streams are mixed with one another by the centrifugal forces.
In the preparation of mono- or polyisocyanates by reaction, by means of a mixing unit of the rotor-stator type, of mono- or polyamines with phosgene dissolved in an organic solvent, the phosgene solution is preferably metered axially to the mixing chamber axis and the amine solution is metered through the rotationally symmetrically arranged inlet openings. This originates from the fact that the inlet of the amine solution is more susceptible to blockages and the amine solution is therefore preferably metered through the inlet openings, to each of which is assigned a pin with which the deposits can be removed.
A disadvantage of the known mixer reactors of the rotor-stator type is that two solutions having viscosities of which the ratio is less than 0.5 or greater than 2 can no longer be mixed adequately if the substance having the lower viscosity is metered axially along the mixing chamber axis, since its centrifugal force as it is transferred through the first rotor disc is no longer sufficient to displace the second substance of higher viscosity as it emerges from the rotationally symmetrically arranged openings in the direction of the outlet of the mixing chamber. As a result, backmixing occurs in the mixing chamber, which is, in particular, problematic on the front plate which modifies the cross-section of the housing and on the inside of the housing walls between the stators. This backmixing leads to caking of solids and to a low content of free isocyanate groups in the mono- or polyisocyanates obtained from the mixer reactor.
A disadvantage of the known mixer reactors of the rotor-stator type is moreover that the concentrations of the dissolved substances cannot be chosen as desired. Unfortunately, in the known mixer reactors, the concentration of the solution of higher viscosity is limited by the fact that its viscosity may not be more than twice the viscosity of the at least second solution. This is a disadvantage in particular in the preparation of mono- or polyisocyanates by reaction of mono- or polyamines with phosgene in organic solvents, since the viscosity of the mono- or polyamine solution changes or varies greatly with the concentration of the mono- or polyamine present in the solution, although the viscosity of the phosgene solution increases only slightly at different phosgene concentrations. Thus, the viscosity of solutions of phosgene in monochlorobenzene (MCB) in the concentration range of from 0 to 80 wt. % at 0° C. is between 0.5 and 1.0 mPa·s (viscosity 0.765 mPa·s and density 1.27 g/l at 0° C. and 50 or 56 wt. %), while the viscosity of a solution of methylenediphenyldiamine (MDA) in monochlorobenzene in the concentration range of from 15 to 65 wt. % at 25° C. is between 1 and 200 mPa·s (see Table 1). On the other hand, the difference in density between the solution is only slight and does not have the effect of making the mixing task difficult.
TABLE 1Density and viscosities of various MDA in MCB solutions at25° C., determined with a Höppler fallingball viscometer from Haake in accordance with DIN 53015Concentration ofTemperatureDensityViscosity inMDA in MCB (%)in ° C.in g/mlmPa s15251.100.9930251.101.8945251.104.2950251.105.4065251.1420.9895251.20>200
Accordingly, it is an object of the present invention to provide a mixer reactor which bypasses the disadvantages mentioned above, and which also ensures a thorough mixing for two flowable substances of widely different viscosities in a quality and speed. In particular, this mixer reactor should allow a process for the preparation of mono- or polyisocyanates with a high content of free isocyanate groups, and therefore makes it possible to use of highly concentrated amine and phosgene solutions.